A bit more clarity after further investigation…
The issue occurs with the ajax item paging within a group, not the “full” paging at the bottom of the page. See attached image. Navigating to the next set of items kills the vlookup data, as does navigating back onto the first set of items that was displayed.
Also of note – if I turn on the ajax options on the library view – ‘Enable Asynchronous Update’ etc – the vLookup data doesn’t show at all, even when the page initially loads.